1. Field of the Invention
The present invention relates to a W-CDMA (Wideband-Code Division Multiple Access) base station using W-CDMA as a communication scheme. More particularly, the present invention relates to a W-CDMA base station in which the configuration of the base station is divided into a base band processing device and remote radio devices installed remote from the base band processing device and in which the base band processing device and the remote radio devices are connected via optical cables.
2. Description of the Related Art
Recently, the W-CDMA communication system using the W-CDMA communication method has been used widely. In the W-CDMA communication system, the range within the reach of radio waves from a base station is a service area. Therefore, there is a possibility that places where it is difficult for radio waves to reach, because these places are remote from the base station, will not be within the coverage. To avoid this inconvenience, the configuration of the base station is divided into a base band processing device and remote radio devices, the remote radio devices are installed remote from the base band processing device, and the base band processing device and remote radio devices are connected by optical cables, thereby keeping places that are remote from the base station within the coverage.
Also, for one base band processing device, a plurality of remote radio devices is installed remote from the base band processing device, thereby increasing the number of cells that can be accessed easily. Further, only by substituting remote radio devices, it is possible to change the configuration of cells that can be accessed easily.
However, in communications between the base band processing device and the remote radio devices connected by optical cables in this way, differences among signal transmission delay times in optical cables will occur which are caused by the different distances of the remote radio devices from the base band processing device, with the distances depending on the locations where each of the remote radio devices are installed. When the lengths of the optical cables between the base band processing device and the respective remote radio devices are equal, there is no problem. However, in cases where the lengths of the optical cables are different, even if downward data is transmitted from the base band processing device to the plurality of remote radio devices at the same time, in the plurality of remote radio devices, the downward data is transmitted to a radio zone while maintaining different timings. A process for correcting the variations is required for diversity transmission, the IPDL (Idle Periods create in the Down Link) service, or the like.
JP-A-2004-222088 discloses a method in which a signal that is to be used for reference timing is transmitted among a plurality of remote radio devices, a time calculated from the reference timing signal and the reception timing of downward data received by the base band processing device are transmitted to the base band processing device, a delay time is calculated in the base band processing device, and variations are corrected.
Also, another method is considered in which, when downward data is transmitted from the base band processing device to the remote radio device, a special synchronous signal is inserted into the downward data, the synchronous signal is detected from the downward data received by the remote radio device, and a delay time is corrected by using the detected synchronous signal and a buffer in which the downward data is temporarily stored.
However, the method disclosed in the above-mentioned patent document has a problem in that the remote radio device requires the preparation of another cable for transmitting and receiving the reference timing signal.
Also, according to the method in which, when downward data is transmitted from the base band processing device to the remote radio device, the special synchronous signal is inserted into the downward data, and a special synchronous signal having a constant period, such as a frame pulse, that is to be inserted into the downward data is required when the downward data is transmitted from the base band processing device to the remote radio device. Further, it is necessary to provide a buffer to absorb the timing for transmission of the downward data to the radio zone so that differences of delay times become zero among the remote radio devices. Accordingly, there is a problem in that the size of device will be increased.